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Originally Posted by ryanwigginton
In the IIA, the way I figured it was by travelling with the battries on charge at ahead 1/3 you only lose one or two knots in speed. But when submerged and not using diesel at all your sub will do 4 knots at ahead 1/3. I guessed this more than makes up for the couple of knots lost while charging batteries. On this basis surely it's an economy. What I've not done is taken into account the distance you lose that one or two knots for (ie charge time). Not sure about different subs.
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This would be correct, or at least a pretty good approximation, if your other engine was also running at 1/3 when charging batteries. But it's running at ahead flank when charging batteries, regardless of what engine setting you pick for propulsion. You can check this yourself, I think most subs have separate engine rpm dials in the control room and in the conning tower.
In this case this means that you do not only lose a few knots when recharging, you also burn diesel at a much higher rate (something like 5 times as much/hour for that engine compared to 1/3 ahead). So instead of using up x+x=2x tons of fuel you use x+5x=6x tons. The distance you gain submerged will make up for some of this, but not all, especially not if you chose the optimal speed setting for your boat which is somewhere inbetween ahead slow and ahead 1/3.
I think some of the confusion regarding this may come from the fact that you can always charge your batteries, even when completely out of fuel. But as long as you still have fuel it will be used when charging batteries.